1. Technical Field of the Invention
The present invention relates to cellular telephone systems and, in particular, to a method for effectuating revisions to a frequency plan that is used for assigning frequencies to transceivers in cells of a cellular telephone system.
2. Description of Related Art
Cellular telephone systems divide a large service area into a number of smaller discrete geographical areas called "cells" each typically ranging in size from about one-half to about twenty kilometers in diameter. Each cell is at least contiguous and/or overlapping with multiple adjacent cells to provide substantially continuous coverage throughout the service area. A base station including a plurality of transceivers capable of operating independently on different assigned radio frequencies is provided for each of the cells. Via the transceivers, the base stations engage in simultaneous communications with plural mobile stations operating within the area of the associated cell. The base stations further communicate via data links and voice trunks with a central control station, commonly referred to as a mobile switching center, which functions to selectively connect telephone calls to the mobile stations through the base stations and, in general, control operation of the system.
Each cell is assigned use of a predetermined set (group) of frequencies from the cellular frequency band for use in providing its control and voice/data (traffic) channels. The assignment is typically made in accordance with a certain frequency plan. The frequencies used for the control and traffic channels assigned to a given cell are preferably spaced apart from each other across the frequency spectrum of the cellular frequency band. This serves to minimize the instances and adverse affects of adjacent channel interference.
Because only a limited number of frequencies are available in the cellular frequency band, the same (for example, group of) frequencies that are assigned by the frequency plan to one cell are also assigned to (i.e., reused by) other cells in distant parts of the service area. Typically, adjacent (i.e., neighbor) cells are not assigned to use the same frequency by the frequency plan. Furthermore, the power levels of the signal transmissions on any given frequency are limited in strength so as to limit propagation beyond the cell area. The foregoing precautions serve to reduce instances of co-channel interference caused by reuse of that same frequency in a distant cell. It is further noted that careful power level and distance assignment also assists in reducing instances of adjacent channel interference.
In spite of the precautions taken by service providers in the frequency plan assignment for a frequency reuse cellular telephone system and in the regulation of system operation, it is known that instances of co-channel interference do occur. This interference may be affected by a number of factors including: terrain irregularities; radio propagation changes; fading; multipath propagation; reflection; existence of human and natural obstructions; the number of available transceivers per cell; and variations in demand. This interference often adversely affects system operation by, for example, degrading voice quality on the traffic channels or interfering with the transmission and reception of control signals on the control channels. Service providers accordingly monitor on a cell by cell basis for instances of adjacent channel and co-channel interference on the assigned frequencies, as well as for instances of relatively low interference on other frequencies, and in response thereto identify potential revisions that could by made to the frequency plan to provide better service by minimizing interference.
A number of algorithms and methodologies are known in the art for either 1) determining from scratch an initial frequency plan for a cellular system (referred to in the art as "starting heuristics") or 2) determining, when starting from a current frequency plan, a better frequency plan for a cellular system (referred to in the art as "improvement heuristics"). See, R. Borndorfer, et al., "Frequency Assignment in Cellular Phone Networks", Konrad-Zuse-Zentrum fur Informationstechnik Berlin, Preprint SC 97-35 (July 1997), Annals of Operations Research 76 (1998), pages 73-93. These and other prior art frequency plan assignment and revision methodologies, while ultimately succeeding in most cases in identifying a better or perhaps near-optimum final frequency plan, do suffer from a number of drawbacks. First, none of these methodologies provide any suggestion to the cellular system operator on how to most efficiently get from the current frequency plan to the final frequency plan in a gradual manner. This is a concern as the proposed revision ignores the operator's operation and maintenance issues relating to being able to effectuate (from a time, equipment, scheduling and manpower perspective) all the retunes necessary to reach the final plan. This is also a concern because during the course of actually performing such a retune to reach the final frequency plan, system performance may be unacceptably degraded if not completed in a timely manner. Second, in situations where the methodology allows the operator to put a cap on the number of retunes allowed to be made in reaching the determined final frequency plan (thus addressing the foregoing operation and maintenance concerns), a concern exists that this new plan may not necessary be the optimum plan. In this regard, the system operator has no a priori knowledge of what would be the optimum plan, and thus has no idea without actually deploying the retune and checking results whether the latest deployed plan is either satisfactory or optimum or even whether a better plan exists and thus the methodology should be run again.